Kearns-Sayre syndrome (KSS) is a rare multisystem mitochondrial disorder associated with progressive external ophthalmoplegia, atypical pigmentary degeneration of the retina, and complete heart block. KSS can lead to a risk of sudden death because of the potential progression of conduction abnormalities such as right or left bundle branch block or complete atrioventricular (AV) block. Here we describe the case of a KSS patient with type I diabetes who experienced syncope in the presence of complete AV block, confirmed by muscular biopsy.